The present invention relates generally to hydro-mechanical and electro-hydro-mechanical systems commonly utilized for positioning mechanical objects (which complete systems will hereinafter be called "control systems").
Most control systems typically utilize a control valve for selectively controlling the application of pressurized hydraulic fluid to the input ports of a power output transducer. The power output transducer is operable for positioning the mechanical objects in response to the pressurized hydraulic fluid acting thereon. In addition, control systems often include feedback elements which are used for establishing a "reference" position for the control valve in order to form a closed-loop servo system. Conventional control systems generally include a pumping apparatus for supplying the pressurized hydraulic fluid at either a substantially constant pressure (hereinafter called "constant pressure systems") or a substantially constant flow rate (hereinafter called "constant flow systems").
Constant pressure systems are the most common. The control valves in constant pressure systems are utilized to control hydraulic fluid flow (hereinafter "load flow") to the input ports of the power output transducers. If the control valve of a constant pressure system is zero-lapped then only leakage amounts of hydraulic fluid will be consumed whenever the control valve is centered Furthermore, if the constant pressure system includes a feedback element for establishing a reference position for the control valve, then the constant pressure system will achieve its final position, with respect to the reference position, substantially without error.
Constant flow systems are typically utilized for vehicular power steering systems. Control valves utilized in such power steering systems are severely under-lapped (i.e., having an open-center configuration) in order to provide passage of the hydraulic fluid without generating an objectionable parasitic pressure loss. This permits the utilization of simple constant displacement pumps with concomitantly minimized average power consumption. However, this also typically results in control characteristics wherein valve deflection primarily regulates load pressure rather than load flow. Furthermore, the load pressure is generally highly nonlinear with respect to valve deflection U.S. Pat. No. 4,460,016 entitled Rotary Servo valve by Haga et al, issued Jul. 17, 1984 discloses the various factors relating to these control characteristics. Thus, such control systems are typically subject to large position error even under relatively light steering loads.
The performance characteristics associated with most conventional vehicular power steering systems substantially conform to a "servo system" of the type subject to a fixed value of steady state position error and a nominally undefined velocity error in response to steady state loads. Alternatively, it is desirable to configure a vehicular power steering system as a servo system having zero position error and a fixed value of velocity error when subjected to steady state loads. In any case, the result (with rotary valve equipped power steering systems) is often a feeling of "play" with concomitant "wander" of a host vehicle when the vehicle is subject to transient load conditions (i.e., such as intermittent side winds or uneven road surfaces).
For this reason some vehicular power steering systems incorporate mechanically interlocking over-ride mechanisms whereby direct coupled manual steering is engaged at light steering loads. However, even such vehicular power steering systems have degraded road feel when compared to manual steering systems. This is because of the parasitic drag associated with various components (i.e., hydraulic power cylinder, seals and the like) and a highly non-linear transition from manual steering to power assisted steering at a selected value of steering load.
Accordingly, the present invention is directed to various control systems having a pumping apparatus which supplies pressurized hydraulic fluid at variable pressures that are selectably related to load pressure. In a first series of embodiments, a pressure regulating valve assembly is used to seIectively by-pass excess hydraulic fluid flow from a pump. This is done in a manner which develops a supply pressure that nominally obeys the equation: EQU P.sub.s =K.sub.1 .vertline.P.sub.L +.DELTA.P
where P.sub.s is the supply pressure, .vertline.P.sub.L .vertline. is the absolute value of load pressure, K is a selected proportionality factor (whose value is usually selected to be 1.0 or very slightly higher in order to compensate for the effects of system leakage and losses) and .DELTA.P is a desired minimum value of P.sub.s present at a zero value of p.sub.L. Thus, the pressure drop through the control valve is maintained at a nominally constant value equal to the difference between the supply pressure and the absolute value of the load pressure.
As will be described below, the nominally constant pressure drop across the control valve results in a more nearly constant, or "stiffer", control valve flow control characteristic with respect to changes in load pressure. In this regard it is similar to a positive feedback technique commonly called "bootstrapping" which is utilized to "stiffen" the output voltage of electronic amplifiers. Because of functional similarity between the bootstrap electronic amplification technique and the control systems to be described hereinbelow, control systems having the pumping apparatus supplying pressurized hydraulic fluid at variable pressures that are selectably related to load pressure will hereinafter to be called "bootstrap control systems". Moreover, bootstrap control systems which use a pressure regulating valve assembly to selectively by-pass excess hydraulic fluid are hereinafter referenced to as "by-pass bootstrap systems".
In a second series of embodiments, the pressure regulating valve assembly, as modified for significantly lower flow, is utilized for selectively controlling the volumetric output of a variable displacement pumping apparatus, with these control systems hereinafter being called "regulated bootstrap systems". This is done in a manner whereby the supply pressure substantially obeys the above equation, and concomitantly, load flow is substantially matched by pump volumetric output.
By-pass and regulated bootstrap systems often incorporate control valves that are zero-lapped or over-lapped (i.e., "closed-center") such that valve leakage is minimal. Since supply pressure is virtually always maintained at a minimum level necessary for control valve function in a bootstrap control system, efficiency is substantially improved when compared to a conventional constant pressure system. In fact, efficiencies in a regulated bootstrap system can even approach those commonly associated with pulse-width modulated electronic servo systems.
Since the difference between the supply pressure and the load pressure is nominally constant regardless of the load value (for positive loads), constant load flow can be substantially maintained without changing the valve opening. Therefore, the value of the error signal is substantially a function of control system output velocity. Thus, in "bootstrap" systems the error signal does not suffer modulation in the face of changing control system load values.
Another feature of the present invention, an improved four-way control valve, is disclosed which can be utilized in the aforementioned bootstrap control systems. The improved four-way control valve features "zero-lapped" or slightly "over-lapped" control orifices (i.e., it is a "closed-center" four-way control valve). Because of its closed-center design, valve deflection of the four-way control valve is primarily determined by load flow rather than load pressure. A primary benefit gained thereby is nominally zero valve deflection for any static load, even when used in constant flow bootstrap systems. Thus, when such constant flow bootstrap systems are incorporated into closed-loop servo systems, the resulting performance substantially conforms to that of the second type of servo system mentioned above.
When the improved four-way control valve of the present invention is utilized in a vehicular power steering system, the primary tactile feedback is related to steering wheel rotational velocity as opposed to steering force. Such tactile feedback can be thought of as "negative rate" feedback. Looked at another way, this primary steering characteristic is actually a positive real value of steering impedance as determined by steering wheel torque divided by steering wheel velocity. This novel steering characteristic is desirable because of its fundamentally stable feel as opposed to a "spring-like" feel (i.e., a negative imaginary value of steering impedance) present with many rotary valve equipped power steering systems.
Accordingly, the present invention is also directed to vehicular power steering systems which overcome the disadvantages associated with convention power steering system via utilization of bootstrap control systems. As such, a first preferred power steering system described herein is a hydro-mechanical power steering system wherein a by-pass bootstrap system is utilized. Yet, another preferred power steering system comprises an electrically powered vehicular power steering system wherein a regulated bootstrap system is utilized. In the electrically powered vehicular power steering system, a controller provides an electrical power signal to a motor driven pump in order to selectively control volumetric hydraulic fluid output.
According to yet another feature of the present invention, an improved "closed-center" four-way control valve is disclosed which can be utilized with virtually any of the various aforementioned bootstrap control systems. The improved four-way control valve is a modified rotary valve having "zero-lapped" or slightly "over-lapped" control orifices. In addition, a plurality of leakage slots are formed in parallel with the main flow distributing slots on the valve spool. The circumferential width of the leakage slots is slightly larger than the circumferential width of the main slots. As such, the leakage slots provide a fluid leakage path between the valve sleeve output slots when the valve spool is rotationally "centered" relative to the valve sleeve. The leakage flow is effective in modifying the characteristic curve of the "closed-center" control valve to nominally provide "open-center" valve characteristics while concomitantly preserving the significant power consumption advantages associated with closed-center four-way control valves.
As a related object, the improved "closed-center" four-way control valve is readily adapted for use in the various hydro-mechanical and electrically powered vehicular power steering systems of the present invention.
Other features, objects and advantages of the present invention will become readily apparent to one skilled in the art upon analysis of the following written description taken in conjunction with the accompanying drawings and appended claims.